In general, improvement of the integration density of semiconductor integrated circuits in recent years has been achieved mainly through a reduction in size of the various circuit patterns. These circuit patterns are presently formed mainly by optical lithography processes using a wafer stepper.
FIG. 1 shows the structure of such a prior art stepper. Mask 14 is illuminated by the light emitted from illumination system 11. An image of mask 14 is projected onto a photoresist film coated on wafer 19 which is the substrate to be exposed through projection system 15. As shown in FIG. 1, illumination system 11 includes a source 10, condenser lens 12, and aperture 13 for specifying the shape and size of the effective source. Projection system 15 includes a projection lens 16, pupil filter 17, and aperture 18 arranged in or near the pupil plane of projection lens 16 to set the numerical aperture (NA) of the lens.
The minimum feature size R of patterns transferable by an optical system is approximately proportional to the wavelength .lambda. of the light used for exposure and inversely proportional to the numerical aperture (NA) of the projection optical system. Therefore, size R is expressed as R=k.sub.1 .lambda./NA, where k.sub.1 is an empirical constant and k.sub.1 =0.61 is referred to as the Rayleigh limit.
As shown by the above expression, the resolution (minimum feature size R) can be increased by decreasing wavelength .lambda. or by increasing the numerical aperture NA. In the past, both approaches have been taken. However, it has recently become difficult to decrease the wavelength further, because of the limited availability of optical materials. Also, lens design issues set a limit to further increases in the numerical aperture. Therefore, pattern dimensions of integrated circuits are now at or near the limit of resolution of the projection exposure system used to define them.
In general, when the pattern dimensions approach the Rayleigh limit, the projected image is no longer a faithful reproduction of the mask pattern shape. This phenomenon is known as optical proximity effects and results in corner rounding, line-end shortening, and line width errors, among other things. To solve this problem, algorithms have been proposed that can be used to pre-distort the mask pattern so that the shape of a projected image takes on the desired shape.
Moreover, approaches have been described which improve the resolution limit of a given optical system, resulting effectively in a decreased value of k.sub.1. Adoption of a phase shifting mask is a typical example of this approach. A phase shifting mask is used to provide a phase difference between adjacent apertures of a conventional mask. Examples of this technique are shown by Mark T. Levenson et al. in an article entitled "Improving Resolution in Photolithography with a Phase-Shifting Mask" in IEEE, Trans. on Electron Devices, Vol. ED-29, No. 12, pp. 1828-1836 (1982)".
A chromeless phase shifting mask method is known as a phase shifting method suitable for the transfer of a fine isolated opaque line pattern, which is needed, for example, for the gate pattern of a logic LSI. A mask used according to this method uses a transparent layer to provide a phase difference of 180.degree. in a transparent area. A very narrow dark line on a bright background is formed along the outline of the transparent layer. This chromeless phase shifting mask method is taught by Toh et al in an article entitled "Chromeless Phase-Shifted Masks: A New Approach to Phase-Shifting Masks" in SPIE vol. 1496 10th Annual Symposium on Microlithography, pp. 27-53 (1990).
An off-axis illumination method and a pupil filtering method are also known methods for improving images. According to the off-axis illumination method, the transmittance of aperture 13 is modified in the illumination system 11 of FIG. 1. One particular embodiment of this method changes the illumination intensity profile so that the transmittance at the margin becomes larger than that of the central portion, which is particularly effective to improve the resolution of a periodic pattern and the depth of focus. The pupil filtering method is a method of performing exposure through a filter (pupil filter) located at the pupil position of a projection lens to locally change the amplitude and phase of the transmitted light. For example, this approach makes it possible to greatly increase the depth of focus of an isolated pattern. The off-axis illumination method is, for example, discussed by Noguchi et al. in an article entitled "Resolution Enhancement of Stepper by Complementary Conjugate Spatial Filter" in SPIE vol. 1674 Optical/Laser Microlithography V, pp. 662-668 (1992). The pupil filtering method is disclosed by Fukuda et al. in the Jpn. J. Appl. Phys. 32 (1993) pp. 5845-5849. Furthermore, it is shown in an article by Orii et al., entitled "Quarter Micron Lithography System with Oblique Illumination and Pupil Filter", SPIE vol. 2197 pp. 854-868 (1994), and shown in European Patent Publication No. 0562133 A1 (1993), that the resolution of a periodic pattern can further be improved by combining the off-axis illumination method and the pupil filtering method.